I don't follow your argument. Verizon's network is federated with AT&T, T-Mobile, Sprint, whomever. Messages can be routed freely between those networks unadulterated. That is what federation means.
To communicate via iMessage you must use Apple's servers. There are no other servers which can accept iMessage messages and forward them back and forth with Apple's server. (Only in a lossy manner, via MMS.) It is not a federated protocol.
Federation promotes competition. Preventing 3rd party access by any mechanism stifles competition.
If iMessage were federated, as in, Beeper could host their own servers which federate with Apple's, I think more of us would side with Apple here.